Existing methods for recovering or extracting gold from gold-containing or gold bearing ores consist of three separately identifiable stages. The first stage is the dissolution of the valuable component i.e. the gold, by forming a solid/liquid mixture i.e. a solution of dissolved gold. The second stage is the separation of the solids and liquids from the liquid/solid mixture to produce a "clean" solution of the liquid containing the dissolved gold or valuable component. The third stage is the extraction of the valuable component from the clean solution. The present invention is an improvement in and/or an alternative for the first two of these three stages. Thus, the present invention relates to an improvement in the steps of dissolving the gold and separating the dissolved gold solution from undissolved raw material.
Existing methods of recovering or extracting gold from solid materials using the above three stages can be generally classified in accordance with the particle size of the solid material bearing the gold that is to be treated by the method.
Firstly, for materials having a largest size of about 1 mm and having a relatively low proportion of fines (which is the term used to describe particles having a size of less than 20 micrometers), one existing method consists of forming a slurry of the solid material to be treated with a suitable leach solution and agitating the slurry so formed for a predetermined time to allow the gold to more or less fully dissolve. The undissolved solid material is then separated from the liquid leach solution by washing, thickening and/or filtering the slurry in order to provide a "clean" solution containing the gold. The separated undissolved solid material is then discarded to waste or may be retreated.
Secondly, for materials which have a largest size of about 500 micrometers and a large proportion of fines, merely dissolving the solid material in a leach solution is not feasible due to the difficulties associated with separating the undissolved solid material from the leach solution to obtain a clean solution of the gold. In the treatment of materials having this particle size range it is additionally necessary to add to the slurry relatively large sized particles of an absorbent substance or substances which absorb the gold from the liquid and which can then be more readily separated from the remaining undissolved non-gold bearing material and from the leach solution. These relatively large sized particles of absorbent materials are subsequently removed from the slurry, often by screening, and then subjected to further processing to extract or otherwise recover the gold.
Thirdly, for material which has a largest size of greater than 1 mm the material to be treated may be placed in a vat and then flooded with leach solution. After a predetermined time the gold-containing solution is drained from the base of the vat providing a clean solution for further processing to recover the valuable component.
Fourthly, for material with a largest size of up to two meters the material may be placed in a heap and the heap irrigated with leach solution. The solution permeates the heap and exits from the base of the heap carrying dissolved gold. The gold can then be recovered from the leach solution.
In any of the above described processes the particle size of the material being treated may be modified. For example, gold bearing material such as ore, is seldom found in a form where the gold is available for direct contact by the leach solution. The more usual situation is where the gold is encased either partially or wholly in the host rock which is usually highly impermeable to liquid and thus initially the leach solution does not have direct access to the gold. As a consequence, it is common practice to comminute the material to a size where the gold is more accessible and can be contacted by the leach solution. This process is known as liberation of the gold.
In cases where the solids/liquid separation stage is achieved by means of drainage, such as in the third and fourth methods described above, it is sometimes necessary to agglomerate the dissolved material prior to recovering the gold which has the effect that any fine particles which are present are agglomerated along with the large particles of material being treated and accordingly are held stationary within the body of the material being leached, thus preventing further recovery.
All of the above processes have one or more limitations., problems or shortcomings and further, they do not always result in the gold being extracted or recovered efficiently or economically.
The first method as described above is limited to the treatment of material which can be readily separated to form a clean solution, such as for example by being readily dewatered by gravitational techniques. Consequently, this process is limited to only those materials of a relatively coarse size only. Additionally this method is also relatively costly in terms of both the capital expenditure required to initially set up the plant and equipment for carrying out this process, and the ongoing operating costs.
Whilst the second method overcomes the dewatering problems associated with the first method it still remains relatively costly to both set up and operate.
The third and fourth method both rely completely on the natural drainage properties of the material being treated in order to achieve satisfactory solid/liquid separation. This is not always effective since the leach solution containing the gold may accumulate in unwanted or in inaccessible locations. In both the heaps of material being treated and the vats containing the material being treated, migration of fine material to either the base of the heap or to a position within the body of the material being leached sometimes prevents adequate drainage through the heap which creates zones where the leach solution becomes stationary and accordingly the method is inefficient in this respect.
Whilst such problems may be partially overcome by agglomeration of the fines, the formation and strengthening of the agglomerates which is sometimes necessary constitutes an additional step in the overall process which further adds to the overall cost of the process.
Therefore, there is a need for a material treatment process in which the leach solutions can be made to pass more efficiently through a wide variety of materials having virtually any particle size distribution. Accordingly, it is one aim of the present invention to provide a process comprising a more efficient leaching step which is applicable to a wide range of materials, irrespective of the properties of the material and which is an improvement over existing methods. The improvement is achieved by applying a potential difference across the solid/liquid mixture so that by the use of electro-osmosis or similar processes liquid is caused to pass through the solid/liquid material at a rate significantly greater than that which can be achieved under gravitational forces alone.
Electro-osmosis as the term is used in the present specification is a term used to describe the phenomenon caused when an electrical potential difference is applied across a solid/liquid mixture. The solid particles present in the slurry or liquid/solid mixture carry a negative surface charge and as a result a positive charge is induced on the liquid molecules immediately adjacent to such particles. This effectively gives the liquid, usually water in the cases of aqueous mixtures, a positive charge. When an electrical field is established between two separated electrodes which are buried in the slurry the solid particles will not undergo appreciable movement because of their relatively close packing, but the water, on the other hand, will be carried towards the negatively charged cathode due to the viscous drag of the migrating positive ions as they move towards the cathode. Such movement is called electro-osmotic flow.
As electro-osmotic flow resulting from electro-osmosis is relatively independent of the pore size of the solid particles of the solid/liquid mixture undergoing electro-osmosis, this technique is applicable to a wide range of size distributions of the solid particles.
Provided that a current can be induced through the solid/liquid mixture, the rate of liquid movement through the mixture and hence through the solid particles will be increased above that which can be achieved under gravitational force alone. The rate of liquid movement achieved is dependent on the size of the potential difference applied, the electrode configuration and the separation between the electrodes. Thus, for a given electrode configuration and a given electrode separation the amount of liquid flow through the mixture can be adjusted by changing the size of the potential difference applied to the mixture.